The present invention relates to the art of induction heating, and more particularly to an inductor for uniformly heating asymmetrically shaped workpieces traveling along a selected path.
This invention is particularly applicable to removing paint from hanger hooks moving along a path and having J-shaped lower ends used to hang parts in spray paint production lines, and it will be described with particular reference thereto; however, it will be appreciated that the invention has much broader applications and can be used for inductively heating a variety of asymmetrical workpieces traveling along a predetermined path.
In electrostatic spray painting operations, the part to be painted must be electrically grounded to the sprayer machinery in order for the paint to be applied uniformly and efficiently. Spray painting is sometimes performed with the painted parts hanging from an overhead conveyor line. The parts are typically suspended from the conveyor using hangers with J-shaped hooks at the bottom. These hangers may also include complimentary J-shaped hooks at the top, being thereby reversible in use. Sprayed paint is normally deposited on the part as well as on the lower hook portion of the hanger. To ensure a proper electrical connection between the painted parts and the spray paint machinery, these hanger hooks must be cleaned prior to reuse with a new part. Induction heating is one method of heating the surface of these hangers in order to remove the deposited paint. Induction heating fractures the bond interface between the paint film and the metal hook such that the paint falls off or can be easily brushed off mechanically. Induction heating also minimizes the energy used and reduces excessive incineration of the paint film, thereby reducing the amount of noxious fumes generated. To remove all the deposited paint, it is desirable that the heat be applied uniformly to all the surfaces upon which paint is deposited. It is well known in the art that uniform inductive heating of the exterior surface of a symmetrical workpiece can be accomplished using inductor coils which completely encircle the workpiece, thereby providing uniform inductive coupling to the entire workpiece surface. An alternative to complete workpiece encirclement, also well known in the art, is the use of flux concentrator materials located with respect to the inductor and the workpiece so as to optimize the coupling of magnetic flux along the workpiece surface. Depending on workpiece geometry, complete workpiece encirclement may be difficult or impossible. Flux concentrators are costly and their performance degrades over time.
Removing paint from hanger hooks in a spray painting operation using induction heating equipment presents problems, one of which is uniform inductive coupling of the asymmetrical portions of the hook shaped hanger workpiece. Locating the induction heating coil segments so as to provide uniform coupling while allowing continuous movement of the workpiece along a path below an overhead conveyor is difficult in this situation.
In one effort to overcome these problems, a prior hanger hook induction heating system disclosed in U.S. Pat. No. 5,617,800 to Moreschi et al. utilizes a four-sided induction heating apparatus including one side directly underneath the workpiece path. This arrangement, however, suffers from the collection of removed paint particles on the lower side of the induction heating apparatus. This accumulation of paint particles can result in non-uniform heating of the hanger workpieces, lower system efficiency, and increased maintenance costs in that the accumulated paint must be removed periodically to avoid non-uniform heating and reduced system efficiency. Consequently, there remains a need for an inductor which provides consistent uniform heating to asymmetrical workpieces suspended from an overhead conveyor apparatus, without the added expense of flux concentrator devices.
Another difficulty in achieving uniform surface heating and the resultant uniformity in paint removal results from hanger workpieces passing the induction heating equipment laterally offset from the normal workpiece path below the conveyor line, whether statically offset or laterally swaying from side to side. In this situation, one side of the hanger workpiece may get too much heat while the other side gets too little. In addition, electrical power supply problems can result if a workpiece contacts the inductor, particularly in electrostatic painting operations where the workpiece is electrically grounded through the conveyor apparatus. The Moreschi system employs lateral stabilizing belts in an attempt to avoid this problem. Use of stabilizing belts or devices, however, results in increased system cost and increased maintenance costs. Accordingly, there remains a need for an efficient, low cost induction heating inductor with an open bottom and which uniformly removes paint from asymmetrically shaped hanger hooks without the need for lateral stabilization belts or devices.